1. Field of the Invention
The present invention relates to a communication by an MPOA (Multi-Protocol Over ATM) system, and more particularly to an MPOA system capable of avoiding Domino Effect in a shortcut communication and its shortcut communication control method, and a shortcut communication control program.
2. Description of the Related Art
The conventional Multi-Protocol Over ATM (hereinafter, referred to as MPOA) system is a communication system using existing layer 3 (network layer) protocols such as Internet Protocol (hereinafter, referred to as IP), and Internetwork Packet Exchange (hereinafter, referred to as IPX) on asynchronous transfer mode (hereinafter, referred to as ATM) networks, and the specifications are defined by the ATM Forum's document AF-MPOA-0087.000.
The feature of the MPOA system is that, when a destination node (target) is in a subnet different from that of a source node of data packets as viewed from the source node, communication is performed by establishing a shortcut Virtual Channel Connection (hereinafter, referred to as VCC) toward the target in an ATM network, without passing through routers, in the case of detecting a data flow (flow of continuous data packets) larger than a predetermined value.
The operation of the conventional MPOA system will be described with reference to FIG. 13. Although ATM switches forming the ATM network are not illustrated in FIG. 13, they are assumed to be placed at proper positions to establish a VCC (including a shortcut VCC) connecting each host and router. As a matter of convenience, IP is used as the layer 3 protocol in the following description, but the protocol is not limited to IP.
In FIG. 13, it is assumed that a host 610 starts to transmit IP packets to a host 650 continuously. The IP packets are forwarded through a router 620, a router 630, and a router 640, toward the host 650. At this time, an MPC 611 that is an MPOA client (hereinafter, referred to as MPC) counts the transmitted IP packets. When the above counted value becomes equal to or larger than a predetermined value in a fixed time (for example, equal to or larger than ten packets in one second), it is judged as “There is a flow”. This is called as flow detection.
The MPC 611 which has detected the flow starts a shortcut communication toward the host 650. In order to start a shortcut communication, it is necessary to establish a shortcut VCC toward the host 650. In order to establish the shortcut VCC, it is necessary to know the ATM address of the host 650. Such an operation that an MPC desiring a shortcut communication obtains the ATM address of a host of a shortcut destination is called as “address resolution”. In other words, the MPC 611 having detected a flow starts the address resolution of the host 650 in order to start a shortcut communication toward the host 650.
The MPC 611 creates an MPOA address resolution request packet which includes the IP address of the host 650 as the target, and transmits it to an MPOA server (hereinafter, referred to as MPS), that is, an MPS 622.
The MPS 622 transmits the MPOA address resolution request packet to an MPS 632 of a router 630 of the next hop toward the target, according to the IP routing table. An MPS like the MPS 622 is called as an ingress MPS. The MPOA address resolution request packet is forwarded one after another toward the target, according to the IP routing table.
An MPS 642 takes out the necessary information from the MPOA address resolution request packet, to create an MPOA cache imposition request packet, and transmits it to an MPC 651 of the target host 650. This is to notify the MPC 651 of Media Access Control (hereinafter, referred to as MAC) header information to be added to the IP packets, when IP packets destined to the host 650 are transmitted from the MPC 511 through a shortcut communication. In the above MAC header information, the MAC address of the host 650 is included as DA (Destination Address), and the MAC address of the router 640 is included as SA (Source Address).
The MPC 651 returns an MPOA cache imposition reply packet to the MPS 642. In the above MPOA cache imposition reply packet, an ATM address for accepting the establishment of a shortcut VCC by the MPC 651 is included.
The MPS 642 takes out the necessary information from the MPOA cache imposition reply packet, to create an MPOA address resolution reply packet, and returns it to the MPS 622 that is the ingress MPS. An MPS like the MPS 642 is called as an egress MPS. The MPOA address resolution reply packet is forwarded one after another, toward the ingress MPS, according to the IP routing table.
When the MPS 622 receives the MPOA address resolution reply packet, it returns the same packet to the MPC 611. Thus, the MPS 611 can obtain the ATM address for establishing a shortcut VCC toward the host 650. The MPOA address resolution request/reply packets and the MPOA cache imposition request/reply packets may be collectively called as MPOA packets.
The MPC 611 establishes a shortcut VCC toward the host 650 by using the above ATM address, and thereafter the IP packets destined to the host 650 are transferred to the shortcut VCC. The MPC 651 receives the IP packets through the shortcut VCC, adds the MAC header previously notified by the MPS 642 through the MPOA cache imposition request packet to the above IP packets, so to create MAC frames. Thus created MAC frames are seemed as if they were transmitted from the router 640 to the host 650.
Thereby, the upper-layer protocol processing of the host 650 may be performed in the same way as in the case where the MPOA system is not introduced.
The above-mentioned conventional MPOA system has a problem of causing Domino Effect.
Hereinafter, the Domino Effect will be described. In FIG. 14, it is assumed that a host 710 starts to transmit IP packets toward a host 750 continuously. The IP packets are forwarded through a router 720, a router 730, and a router 740, to the host 750. An MPC 711 which has detected a flow toward the host 750 transmits an MPOA address resolution request packet to an MPS 722 in order to start a shortcut communication toward the host 750.
Here, the MPC 711 counts the flow, and simultaneously an MPC 721 of the router 720 and an MPC 731 of the router 730 count the flow. Accordingly, the MPC 721 and the MPC 731 detect the flow toward the host 750, in parallel with the MPC 711. Therefore, the MPC 721 and the MPC 731 transmit the MPOA address resolution request packets to the MPS 732 and an MPS 742 respectively, in order to start a shortcut communication toward the host 750. This is called as the Domino Effect in the MPOA system.
When the Domino Effect occurs, a redundant shortcut VCC is established. This is because both “the ATM address of a source MPC” in the respective MPOA address resolution request packets from the MPC 721 and the MPC 731 are different from that of the MPC 711, the MPS 742 that is the egress MPS regards them as different flows, and the MPS 742 processes the respective MPOA address resolution request packets from the MPC 721 and the MPC 731 in the same way as it processes the MPOA address resolution request packet from the MPC 711.
FIG. 15 shows the state where redundant shortcut VCCs have been established as a result of occurrence of the Domino Effect. In this example, although shortcut VCCs are established respectively from the router 720 and the router 730 toward the host 750, these shortcut VCCs will be never used. This is because the IP packets from the host 710 to the host 750 are all transferred on the shortcut VCC toward the host 750, in the host 710.
As the conventional technique for avoiding the Domino Effect, there is a method in which, in each router, shortcut processing is not performed on a data flow coming from an ATM network and going to an ATM network again. This technique has been described by the specifications of NHRP (RFC2332, IETF) used in the MPOA system as the address resolution protocol.
This conventional technique, however, has a problem that even a flow naturally to be targeted for a shortcut is excluded.